1. Field of the Invention
This invention relates to a process for coating a metal surface with a powder coating containing an acrylic copolymer flow control agent.
2. Description of the Related Art
Low-solvent or solventless paint systems are becoming increasingly more important. Paint systems such as these include the so-called powder coatings, i.e. solventless powder-form paints which are generally applied to substrates by electrostatic application and are hardened at relatively high temperatures. The success of this technique presupposes the presence of all components in powder form.
To obtain satisfactory surfaces with powder coatings, flow control agents are added. Flow control agents are generally liquid polyacrylate-based products which, basically, are incorporated in the powder coatings in two different ways, namely: (1) in the form of a so-called master batch, the liquid flow control agent being fused into the binder component of the powder coating; the solidified melt is then broken up and made up for subsequent processing; (2) in the form of a liquid coated onto an inert support, for example silica gel.
The disadvantage of the master batch technique is that the formulation of powder coatings is limited because the binder of the master batch has to be identical or at least compatible with the principal binder of the formulation. Flow control agents coated onto a support can lead to a reduction in the gloss of the coating and to a reduction in transparency in the case of clear powder coatings and, under adverse conditions, to a reduction in the stability of the powder coating in storage.
The present invention relates to solid, powder-form flow control agents which require neither an inert support nor incorporation by the master batch technique, so that the disadvantages mentioned above can be avoided.
The present invention is applicable to standard types of powder coatings as described in Ullmann's Enzyklopadie der technischen Chemie, 4th Edition, Vol. 15, pages 660-664, Verlag Chemie Weinheim (1978), to the contents of which reference is expressly made. The present invention is particularly suitable for so-call ed electrostatic powders based on epoxy resins, saturated polyester resins and polyacrylates, cf. loc. cit., pages 660-661, number 3.4.1. The acrylic copolymers used as flow control agents in the present invention are disclosed in U.S. patent application Ser. No. 07/326,598 filed on March 3, 1989, now U.S. Pat. No. 5,006,621, patented Apr. 9, 1991.